The present invention relates to the preparation of drilling muds, pulps and suspensions, and, in particular to methods for controlling the dispersing of solids in drilling muds.
The present invention can be used to good advantage in the petroleum and gas industries for preparing drilling muds.
The dispersing of solids is a critical process operation, as it governs to a great degree the quality of the prepared drilling mud. An effective dispersing of clays reduces their consumption through greater yield of drilling mud per unit mass of clay powder. A reduction in the total amount of solids in the mud while retaining the colloidal state factor value raises the penetration rate.
Dispersing is employed for preparing drilling muds based on clay materials, wherever the clay powders possess a low colloidal state factor.
The dispersing of solids in a liquid phase involves physicochemical changes affecting the state and the properties of both the solid and the liquid phases. A measure of the dispersing effectiveness is the fineness of the solid phase, described in terms of the concentration of particle thereof in the mud.
It is common knowledge that the most active clay particles as regards the drilling mud structure formation are ones measuring 5 .mu.m and less across. The dispersing consists in converting a maximum possible proportion of the solid phase to colloidal particles less than 5 .mu.m across.
However, of major importance in the formation of the coagulation structure is not only the size of the colloidal particles, but also their number, i.e. the concentration of particles. In practice, a small number of solid colloid particles fails to ensure a strong coagulation structure, whereas their excess number may sharply increase the gel strength and the flow resistance of the mud and so bring down the penetration rate and hamper the removal of cuttings from the drilling mud. The optimum concentration of particles of the solid phase in a drilling mud varies with the type thereof, so that the concentration of particles is one of the main parameters characterizing the drilling mud solid phase dispersion.
There is known a method for monitoring the dispersing of solids in drilling mud by measuring the electric parameters thereof. The method consists essentially in placing electrodes in a liquid stream and measuring the difference of potentials across electrodes, as particles move together with the stream and with respect to the liquid.
This phenomenon, termed the electrokinetic effect, is due to a double electric layer at the solid-particle-liquid interface, as the solid particle and the liquid possess a definite charge. The electrokinetic potential in the drilling mud stream varies with the change in the concentration of the solid phase particles. The greater the concentration of particles in the liquid, the greater is the electrokinetic potential, as the difference in the potentials is governed by the charge transmitted by the particles to the electrodes of the measuring apparatus. The greater the number of these particles, the greater is the potential difference.
Therefore, the determination of the electric parameters of a drilling mud, in particular, of the electrokinetic potential thereof, provides information on the solid phase fineness, which is taken to indicate the degree of dispersion.
The prior art method makes it possible to control the variation of the amount of the solid phase in the liquid, but fails to ensure a continuous control of the dispersing of solids in drilling mud and to determine the end of the solid phase dispersion, this providing no means for preparing the drilling mud under optimum conditions. In addition, the method necessitates a system of electrodes with recorders, their erection, setting-up and servicing, this involving extra labour and power consumption.
There is also known a method for controlling the dispersing of solids in drilling, wherein the concentration of solids is measured by means of a vibration-type pickup consisting of an oscillation exciter formed with a mechanical oscillation converter and a detecting element having a grid frame, the plane of which is perpendicular to the direction of oscillations. The grid frame of the vibration pickup is rigidly connected to the oscillation exciter and is immersed into the drilling mud stream at the right angle to its flow. If solid phase particles in the mud are larger than the clear openings in the grid frame, they block the apertures of the grid, thereby sharply increasing the resistance the detecting element offers to flow. The greater the concentration of solid phase particles in the mud, the greater is the resistance to flow of the grid, this reducing the detecting element oscillation amplitude, the magnitude of which is a measure of the concentration of solids in the liquid phase. The greater the concentration of particles in the mud, the higher is the degree of dispersion.
Therefore, the variation of the amplitude of oscillations of the detecting elements indicates the variation in the concentration of particles in the liquid phase. However, this known method also fails to provide a continuous control of the dispersing of solids in drilling mud and the determination of the optimum solid phase dispering time, particularly where the solid phase is clays of various mineralogical compositions having different starting strengths of clay particles. The various types of clays present different colloidal state factors with the effect that the specified size of the particles being dispersed has different values. Therefore, the application of this known method requires to change the detecting element with each type of clay, a suitably apertured frame being selected each time. This results in a periodic discontinuance of the dispersing process and the re-setting of the dispersing apparatus operating parameters, leads to additional consumptions of labor and power and lowers the throughput capacity of the dispersing apparatus. Dispersing under non-optimum conditions would bring about either an over-dispersion of the solid phase in the liquid phase with attendant excess power consumption or insufficient dispersion of large and tough particles with, in both cases, resultant instability and deterioration of drilling mud properties.